Stainless separator for fuel cell and method of manufacturing the same

ABSTRACT

A stainless steel separator for fuel cells and a method of manufacturing the same are disclosed. The method includes preparing a stainless steel sheet as a matrix, performing surface modification on a surface of the stainless steel sheet to form a Cr-rich passive film having a comparatively increased amount of Cr in a superficial layer of the stainless steel sheet by decreasing an amount of Fe in the superficial layer of the stainless steel sheet, and forming a coating layer on the surface of the surface-modified stainless steel sheet. The coating layer is one selected from a metal nitride layer (MN x ), a metal/metal nitride layer (M/MN x ), a metal carbide layer (MC y ), and a metal boride layer (MB z ) (where 0.5≦x≦1, 0.42≦y≦1, 0.5≦z≦2).

CROSS-REFERENCE TO RELATED APPLICATIONS

This present application is a Divisional Application of U.S. Ser. No.12/426,150 filed Apr. 17, 2009, which claims the benefit from KoreanPatent Application No. 10-2008-0037916, filed on Apr. 23, 2008 and No.10-2008-0041799, filed on May 6, 2008 in the KIPO (Korean IntellectualProperty Office).

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a stainless steel separator for fuelcells and a method of manufacturing the same. More particularly, thepresent invention relates to a stainless steel separator for fuel cellsand a method of manufacturing the same, which is used for polymerelectrolyte fuel cells (PEMFCs) and exhibits superior corrosionresistance, electrical conductivity, and durability.

2. Description of the Related Art

Since a unit cell of a fuel cell stack generates too low a voltage to beused alone in practice, the fuel cell stack generally includes severalto several hundred unit cells stacked therein. When stacking the unitcells, a separator or bipolar plate is used to facilitate electricalconnection between the unit cells while separating a reaction gas.

The bipolar plate is an essential component of the fuel cell along witha membrane electrode assembly (MEA) and has a variety of functions, suchas structural support for the MEA and gas diffusion layers (GDLs),collection and transfer of current, transmission and removal of reactiongas, transmission of water coolant for removing reaction heat, etc.

Hence, it is necessary for materials of the bipolar plate to haveexcellent electrical and thermal conductivity, air-tightness, chemicalstability, and the like.

Graphite-based materials and composite graphite materials composed ofresin and graphite are employed as the materials for the bipolar plate.

However, the graphite-based material has lower strength andair-tightness than metallic materials, and demands high manufacturingcosts irrespective of low productivity when applied to the bipolarplate. Recently, metallic bipolar plates have been actively investigatedto overcome such problems of the graphite bipolar plate.

When the bipolar plate is made of a metallic material, there are manymerits in that volume and weight reduction of a fuel cell stack can beaccomplished via thickness reduction of the bipolar plate, and in thatthe bipolar plate can be fabricated by stamping and the like, therebyensuring mass production of the bipolar plates.

However, the metallic material inevitably undergoes corrosion during useof the fuel cell, causing contamination of the MEA and performancedeterioration of the fuel cell stack. Further, a thick oxide film can begrown on the metal surface after extended use of the fuel cell, therebycausing an increase in internal resistance of the fuel cell.

Stainless steel, titanium alloys, aluminum alloys, nickel alloys, andthe like are proposed as candidate materials for the bipolar plate ofthe fuel cell. Particularly, stainless steel has received attention dueto its low price and good corrosion resistance, but further improvementsin corrosion resistance and electrical conductivity are still needed.

SUMMARY OF THE INVENTION

The present invention is conceived to solve the problems of the relatedart as described above, and an aspect of the present invention is toprovide a method of manufacturing a stainless steel separator for fuelcells that has corrosion resistance and contact resistance satisfyingthe standards of the Department of Energy (DOE) not only at an initialstage but also after exposure to high temperature-high humidityconditions in the fuel cell for a long duration.

Another aspect of the present invention is to provide a stainless steelseparator manufactured by the method.

According to one embodiment of the present invention, a method ofmanufacturing a stainless steel separator for fuel cells includes:preparing a stainless steel sheet as a matrix; performing surfacemodification on a surface of the stainless steel sheet to form a Cr-richpassive film having a comparatively increased amount of Cr in asuperficial layer of the stainless steel sheet by decreasing an amountof Fe in the superficial layer of the stainless steel sheet; and forminga coating layer on the surface of the surface-modified stainless steelsheet, the coating layer being one selected from a metal nitride layer(MN_(x)), a metal/metal nitride layer (M/MN_(x)), a metal carbide layer(MC_(y)), and a metal boride layer (MB_(z)) (where 0.5≦x≦1, 0.42≦y≦1,0.5≦z≦2).

According to another embodiment of the present invention, a method ofmanufacturing a stainless steel separator for fuel cells includes:preparing a stainless steel sheet as a matrix; performing surfacemodification on a surface of the stainless steel sheet to form a Cr-richpassive film having a comparatively increased amount of Cr in asuperficial layer of the stainless steel sheet by decreasing an amountof Fe in the superficial layer of the stainless steel sheet; andheat-treating the surface-modified stainless steel sheet at 100˜300° C.under vacuum, in air or in an inert gas atmosphere.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects, features and advantages of the presentinvention will become apparent from the following description ofexemplary embodiments given in conjunction with the accompanyingdrawings, in which:

FIG. 1 is a flowchart of a method of manufacturing a stainless steelseparator according to one embodiment of the present invention;

FIGS. 2 to 4 are perspective views illustrating the respective steps ofthe method shown in FIG. 1;

FIG. 5 is a cross-sectional view of a contact resistance tester formeasuring contact resistance of a stainless steel sheet according to thepresent invention;

FIG. 6 is a graph depicting results of evaluating corrosion resistanceof Example 1 and Comparative Examples 2 and 3 in a simulated fuel cellenvironment;

FIG. 7 is a graph depicting results of evaluating contact resistance ofExamples 1, 4, 8 to 18 and Comparative Examples 1 and 2 in a simulatedfuel cell environment;

FIG. 8 is a graph depicting results of evaluating long-term durabilityof Examples 1, 4, 8 to 18 and Comparative Example 1 in a simulated fuelcell environment;

FIG. 9 is a flowchart of a method of manufacturing a stainless steelseparator according to another embodiment of the present invention;

FIG. 10 is a graph depicting results of evaluating contact resistance ofExamples 19, 21, 23 and 26, and Comparative Examples 4, 6 to 8 in asimulated fuel cell environment;

FIG. 11 is a graph depicting results of evaluating corrosion currentdensity of Examples 19 and 21, and Comparative Example 4 exposed to asimulated fuel cell environment for 2,000 hours; and

FIG. 12 is a graph depicting results of evaluating long-term durabilityof Examples 19, 21, 23 and 26, and Comparative Example 4, 6 to 8 in asimulated fuel cell environment.

DETAILED DESCRIPTION OF THE EMBODIMENT

Hereinafter, exemplary embodiments of the present invention will bedescribed in detail with reference to the accompanying drawings.

However, it should be noted that the present invention is not limited tothe embodiments and can be realized in various forms, and that thefollowing embodiments are given by way of illustration to provide athorough understanding of the invention to those skilled in the art.Therefore, the present invention is defined only by the accompanyingclaims. Like elements will be denoted by like reference numeralsthroughout the specification

Further, it should be noted that the drawings are not to precise scaleand may be exaggerated in thickness of layers, films and/or regions fordescriptive convenience and clarity only. When a certain film or layeris described as being formed “on” another film or layer, the certainfilm or layer may be disposed directly on the other film or layer, ormay be disposed above the other film or layer with a third film or layerinterposed therebetween.

FIG. 1 is a flowchart of a method of manufacturing a stainless steelseparator according to one embodiment of the present invention, andFIGS. 2 to 5 are perspective views illustrating the respective steps ofthe method shown in FIG. 1.

To manufacture a stainless steel separator according to the embodimentof the invention, a stainless steel sheet 200 is prepared in S110, asshown in FIG. 2.

In this embodiment, the stainless steel sheet 200 is a stainless steelsheet which is readily available in the marketplace and contains 16˜28wt % chromium. Alternatively, the stainless steel sheet may containabout 18 wt % chromium.

Specifically, a matrix of the stainless steel sheet 200 is a stainlesssteel sheet that comprises 0.08 wt % or less carbon (C), 16˜28 wt %chromium (Cr), 0.1˜20 wt % nickel (Ni), 0.1˜6 wt % molybdenum (Mo),0.1˜5 wt % tungsten (W), 0.1˜2 wt % tin (Sn), 0.1˜2 wt % copper (Cu),and the balance of iron (Fe) and unavoidable impurities. Morespecifically, the stainless steel sheet is an austenite stainless steelsuch as SUS 316L 0.2t.

This operation may include a cleaning process for removing impuritiesfrom the surface of the stainless steel sheet 200 using acid and alkalidegreasers before performing subsequent surface modification andformation of a coating layer.

Next, as shown in FIG. 3, the surface of the stainless steel sheet 200is subjected to surface modification in S120.

Although the stainless steel sheet 200 contains chromium and nickelcomponents exhibiting high corrosion resistance, the stainless steelsheet 200 is mainly composed of iron (Fe).

As a result, in a natural state, the stainless steel sheet 200 tends toreact with oxygen in air to form an oxide film on the surface of thestainless steel sheet. Here, since the oxide film is an insulator, itcan cause deterioration of the overall electrical conductivity of thestainless steel sheet 200.

Therefore, there is a need for surface modification on the surface ofthe stainless steel sheet which undergoes deterioration in corrosionresistance.

In other words, the surface modification is performed for selectivelyetching only the iron component (Fe) in a superficial layer of thestainless steel sheet 200.

After the surface modification, the surface of the stainless steel sheet200 becomes a Cr-rich passive film 210. The Cr-rich passive film 210contains 20˜75 wt % chromium and 30 wt % or less iron, and has a(Cr+Ni)/Fe ratio of 1 or more as expressed by a ratio of main componentsin the Cr-rich passive film 210.

Here, the selective metal dissolution can be accomplished because ironoxide in the superficial oxide film can be easily dissolved in an acidwhereas chromium oxide therein is more stable than the iron oxide anddoes not easily dissolve in acids.

Next, a solution and conditions for the surface modification will bedescribed.

A surface modification solution comprises 5˜20 wt % pure nitric acid(HNO₃), 2˜15 wt % pure sulfuric acid (H₂SO₄), and the balance of water.The surface modification may be performed at 50˜80° C. for an immersionduration of 30 seconds to 30 minutes or less. Here, the surfacemodification may be performed for 30 seconds to 10 minutes or less whileadjusting the concentrations of the nitric acid and the sulfuric acid inconsideration of productivity according to treatment duration.

According to one embodiment of this invention, the surface modificationsolution may be prepared by adding one or both of oxalic acid (C₂H₂O₄)and hydrogen peroxide (H₂O₂) to the aforementioned surface modificationsolution (nitric acid+sulfuric acid) to accelerate a metal dissolutionrate on the surface of the stainless steel sheet.

Further, for the surface modification, an electrochemical process may becarried out by applying an SHE potential of greater than 0.0 to 1.0 V tothe stainless steel sheet which has been immersed in the surfacemodification solution comprising sulfuric acid (H₂SO₄), thereby enablingselective dissolution of Fe in a further reduced period of time.

With the surface modification, a large amount of Fe and a part of Nicontent are selectively dissolved to reduce the amount of Fe in thesuperficial layer of the stainless steel sheet without substantiallydissolving chromium (Cr) therein, so that the chromium and nickelcomponents are concentrated on the superficial layer of the stainlesssteel sheet.

After the surface modification, the Cr-rich passive film 210 may have athickness of 5˜100 nm.

Next, a coating layer is formed on the Cr-rich passive film 210 in S130,as shown in FIGS. 4 and 5.

The coating layer 220 may be selected from a metal nitride layer(MN_(x)), a metal/metal nitride layer (M/MN_(x)), a metal carbide layer(MC_(y)), and a metal boride layer (MB_(z)). The formation of thecoating layer is performed for the following reasons.

When the stainless steel sheet 200 is modified, the Cr-rich passive film210 is formed on the stainless steel sheet as described above, therebyensuring superior corrosion resistance and electrical conductivity at aninitial stage.

However, when a surface-modified stainless steel separator is exposedfor long durations to high temperature-high humidity conditions of afuel cell, the passive film is gradually thickened. Here, since thepassive film mainly consists of metallic oxides, the stainless steelseparator undergoes deterioration in electrical conductivity after apredetermined operating period even though the corrosion resistance ismaintained.

Accordingly, the coating layer 220 selected from the metal nitride layer(MN_(x)), the metal/metal nitride layer (M/MN_(x)), the metal carbidelayer (MC_(y)) and the metal boride layer (MB_(z)) and having bothsuperior corrosion resistance and superior electrical conductivity isformed on the Cr-rich passive film 210, so that the separator for thefuel cell can be prepared to have superior corrosion resistance andsuperior electrical conductivity not only at an initial operating stagebut also after long-term operation.

Here, metal (M) constituting the coating layer 220, which is selectedfrom the metal nitride layer (MN_(x)), the metal/metal nitride layer(M/MN_(x)), the metal carbide layer (MC_(y)) and the metal boride layer(MB_(z)), may be selected from transition metals, which have bothsuperior corrosion resistance and superior electrical conductivity innitride form. Specifically, the metal may selected from chromium (Cr),titanium (Ti), zirconium (Zr), and tungsten (W) (where 0.5≦x≦1,0.42≦y≦1, 0.5≦z≦2).

The coating layer 220 may have a thickness of 30˜300 nm, and preferablya thickness of 30˜100 nm. The coating layer having a thickness less than30 nm provides insignificant effects, whereas the coating layer having athickness greater than 300 nm deteriorates productivity due to a highprice of a metal target and a long-term process.

The coating layer 220 selected from the metal nitride layer (MN_(x)),the metal/metal nitride layer (M/MN_(x)), the metal carbide layer(MC_(y)) and the metal boride layer (MB_(z)) may be obtained, withoutlimitation, by arc ion plating or physical vapor deposition such assputtering and the like.

In this embodiment, since reactive sputtering permits easy control ofthe process, the reactive sputtering is used for forming the coatinglayer 220 selected from the metal nitride layer (MN_(x)), themetal/metal nitride layer (M/MN_(x)), the metal carbide layer (MC_(y))and the metal boride layer (MB_(z)).

Examples of the metal (M) constituting the coating layer 220 selectedfrom the metal nitride layer (MN_(x)), the metal/metal nitride layer(M/MN_(x)), the metal carbide layer (MC_(y)) and the metal boride layer(MB_(z)) include chromium (Cr), titanium (Ti), zirconium (Zr), andtungsten (W).

Although sputtering is used in this embodiment, other processes may alsobe used to form the coating layer 220.

To form the coating layer 220, a metal target having a purity of 99.99%or more may be used as a sputtering target.

A technique for forming the coating layer by sputtering will bedescribed in more detail. After the stainless steel sheet 200 and themetal target are loaded in a sputtering chamber, sputtering is performedin an atmosphere of argon and nitrogen (Ar+N₂) gas to form the coatinglayer 220 on the passive film 210 of the stainless steel sheet.

When forming a coating layer consisting of two layers of M/MNx, argongas is supplied alone to form a metal layer (M), followed by supplyingthe argon and nitrogen (Ar+N2) gas to form a MN_(x) layer continuouswith the metal layer (M), so that the two layer of M/MN_(x) can becontinuously formed.

As such, in this process, the sputtering is performed in an argon gasatmosphere when forming the metal layer (M), and is then performed inthe atmosphere of argon and nitrogen (Ar+N₂) gas when forming the metalnitride layer (MN_(x)).

In more detail, referring to FIG. 4, the coating layer 220 is formed ina continuous film shape on the passive film 210.

EXAMPLES AND COMPARATIVE EXAMPLES

Next, a description of the present invention will be given withreference to inventive and comparative examples to show that a stainlesssteel separator for fuel cells manufactured by the method according tothe embodiment of this invention has excellent corrosion resistance andcontact resistance. A description of details apparent to those skilledin the art will be omitted herein.

TABLE 1 CrN coating Pickling Coating Corrosion Temp. Time layerThickness rate CR Process (° C.) (min) Composition design (nm) (μA/cm²)(mΩ cm²) E1 IM 60 3 15% HNO₃ + 10% CrN 30 0.75 14.6 H₂SO₄ E2 IM 60 3 5%HNO₃ + 5% CrN 50 0.72 15.1 H₂SO₄ + 5% Oxalic E3 IM 60 3 5% HNO₃ + 5% CrN50 0.73 14.9 H₂SO₄ + 5% H₂O₂ E4 IM 60 3 10% HNO₃ + 5% Cr/CrN 300 0.5814.3 H₂SO₄ + 5% H₂O₂ multilayer E5 EC 60 100 1M H₂SO₄ CrN 50 0.76 14.6E6 EC 60 100 1M H₂SO₄ Cr/CrN 100 0.79 15.2 multilayer E7 EC 60 100 1MH₂SO₄ CrN 100 0.82 14.3 E8 EC 60 100 1M H₂SO₄ TiN 100 0.74 14.9 E =Example, IM = Immersion process, EC = Electrochemical process, CR:Contact resistance

Tables 1, 2 and 3 show corrosion currents and contact resistances ofstainless steel separators of Examples 1 to 18 and Comparative Examplesprepared using stainless steel 316 L as matrices of the stainless steelseparators via the immersion process and the electrochemical processunder different conditions for surface modification (temperature, time,current density, and solution composition) and under differentconditions (kind, design, and thickness of coating layer) in formationof a coating layer.

TABLE 2 CrN coating Pickling Coating Corrosion Temp. Time layerThickness rate CR Process (° C.) (min) Composition design (nm) (μA/cm²)(mΩ cm²) E9 EC 60 100 1M H₂SO₄ ZrN 100 0.81 13.8 E10 IM 60 3 15% HNO₃ +10% Cr₂N 100 0.89 13.9 H₂SO₄ E11 IM 60 3 15% HNO₃ + 10% TiC 100 0.8516.1 H₂SO₄ E12 IM 60 3 15% HNO₃ + 10% ZrC 100 0.81 16.6 H₂SO₄ E13 IM 603 15% HNO₃ + 10% Cr₃C₂ 100 0.83 16.9 H₂SO₄ E14 IM 60 3 15% HNO₃ + 10%Cr₇C₃ 100 0.84 16.6 H₂SO₄ E15 IM 60 3 15% HNO₃ + 10% CrB₂ 100 0.92 15.9H₂SO₄ E16 IM 60 3 15% HNO₃ + 10% TiB₂ 100 0.85 16.1 H₂SO₄ E = Example,IM = Immersion process, EC = Electrochemical process, CR: Contactresistance

Specifically, Examples 1 to 7, and 10 were subjected to both surfacemodification and formation of a chromium nitride layer (CrN or Cr₂N)(Examples 4 and 6 had Cr/CrN multiple layers). Examples 8, 9 and 11 to18 were formed with coating layers of titanium nitride, titanium carbideand titanium boride (TiN, TiC and TiB₂), zirconium nitride, zirconiumcarbide and zirconium boride (ZrN, ZrC and ZrB₂), chromium carbide andchromium boride (Cr₃C₂, Cr₇C₃ or CrB₂), and tungsten carbide (WC),respectively. Comparative Example 1 was formed with a coating layer ofchromium nitride (CrN) having a thickness of 15 nm which is greater thanthe thickness of the coating layer 220 according to the presentinvention. Comparative Example 2 was subjected only to surfacemodification without the formation of the coating layer, and ComparativeExample 3 was formed with only a chromium nitride (CrN) layer withoutthe surface modification.

TABLE 3 Pickling CrN coating Corrosion Temp. Time Coating Thickness rateCR Process (° C.) (min) Composition layer design (nm) (μA/cm²) (mΩ cm²)E17 IM 60 3 15% HNO₃ + 10% ZrB₂ 100 0.81 16.6 H₂SO₄ E18 IM 60 3 15%HNO₃ + 10% WC 100 0.86 17.6 H₂SO₄ CE1 IM 60 3 15% HNO₃ + 10% CrN  150.94 17.3 H₂SO₄ CE2 EC 60 100  1M H₂SO₄ — — 0.95 17.5 CE3 — — — — CrN 30 2.3 35 E = Example, IM = Immersion process, EC = Electrochemicalprocess, CR: Contact resistance

1. Measurement of Contact Resistance

FIG. 5 is a cross-sectional view of a contact resistance tester formeasuring contact resistance of a stainless steel separator according toone embodiment of the present invention.

Referring to FIG. 5, in order to obtain optimized parameters for cellassembly for measurement of contact resistance of a stainless steelsheet 500, a modified Davies method was used to measure contactresistance between stainless steel (SS) and two pieces of carbon paper.

The contact resistance was measured based on the principle of measuringfour-wire current-voltage via a contact resistance tester available fromModel IM6 from Zahner Inc.

Measurement of the contact resistance was performed by application of DC2 A and AC 0.2 A to a measuring target in a constant current mode at afrequency in the range of 10 kHz to 10 mHz.

The carbon paper was 10 BB available from SGL Inc.

In the contact resistance tester 50, a sample 500 was disposed betweentwo pieces of carbon paper 520 and copper plates 510 connected to both acurrent supplier 530 and a voltage tester 540.

Voltage was measured by applying DC 2 A/AC 0.2 A to the sample 500 usinga current supplier 530 (Model IM6 from Zahner Inc.).

Then, the sample 500, carbon paper 520, and copper plates 510 werecompressed to form a stacked structure from both copper plates 510 ofthe contact resistance tester 50 using a pressure regulator (Model No.5566 from Instron Inc., compression maintaining test). Using thepressure regulator, a pressure of 50˜150 N/cm² was applied to thecontact resistance tester 50.

The contact resistances of samples 500, that is, stainless steel sheets,of the inventive and comparative examples shown in Tables 1 and 2 weremeasured using the contact resistance tester 50 prepared as describedabove.

2. Measurement of Corrosion Current Density

A corrosion current density of the stainless steel sheet according tothe present invention was measured using EG&G Model No. 273A as acorrosion current tester. Tests for corrosion durability were performedin a simulated environment of a polymer electrolyte fuel cell (PEFC).

After being etched at 80° C. using 0.1N H₂SO₄+2 ppm HF as an etchingsolution, the samples were subjected to O₂ bubbles for 1 hour, and thecorrosion current density thereof was measured at an open circuitpotential (OCP) of −0.25V˜1V vs. SCE.

Other properties were measured at −0.24V vs. SCE for a PEFC anodeenvironment and at 0.6V vs. SCE for a PEFC cathode environment.

Here, the measured properties were evaluated based on data of corrosioncurrent at 0.6V vs. SCE in a simulated cathode environment of a fuelcell.

The anode environment is an environment in which hydrogen is split intohydrogen ions and electrons while passing through a membrane electrodeassembly (MEA), and the cathode environment is an environment in whichoxygen combines with the hydrogen ions to produce water while passingthrough the MEA.

Since the cathode environment has a high potential and is verycorrosive, it is desirable that the corrosion resistance be tested inthe cathode environment.

Further, it is desirable that the stainless steel sheet have a corrosioncurrent density of 1 μA/cm² or less for application to the PEFC.

3. Analysis of Measurement Results of Corrosion Current Density andContact Resistance

It can be seen from Tables 1 to 3 that, when the samples were subjectedto surface modification and were formed with metal coating layers as inthe inventive examples, the samples had a corrosion current density of0.5˜1.0 μA/cm² and a contact resistance of 13˜18 mΩ·cm².

Comparative Example 1 having a 15 nm thick chromium nitride layer formedafter the surface modification had a contact resistance of 17.4 mΩ·cm²and a corrosion current density of 0.94 μA/cm². Comparative Example 2subjected only to the surface modification had a contact resistance of17.5 mΩ·cm² and a corrosion current density of 0.95 μA/cm². ComparativeExample 3 formed with a chromium nitride layer without the surfacemodification had a contact resistance of 35 mΩ·cm² and a corrosioncurrent density of 2.3 μA/cm².

Although the inventive examples and comparative examples had contactresistances and corrosion current densities satisfying the standards ofthe DOE, these values were merely initial values before long-termoperation of a fuel cell, and it can be seen that a difference betweenthe inventive examples and comparative examples is significant inevaluation of long-term durability of the fuel cell described below.

4. Evaluation and Result of Corrosion Resistance and Contact Resistancein Simulated Fuel Cell Environment

(1) Evaluation of Corrosion Resistance and Contact Resistance in aSimulated Environment

For a simulated fuel cell environment to test a stainless steelseparator according to one embodiment of this invention, EG&G Model No.273A was used. After being immersed in 0.1N H₂SO₄+2 ppm HF at 80° C.,samples were subjected to O₂ bubbles for 1 hour, followed by applicationof a constant voltage of 0.6V vs. SCE. After applying the constantvoltage for a predetermined duration, the corrosion resistance andcontact resistance of each sample were measured. While repeating thisoperation, the variation in corrosion resistance and contact resistancein the simulated fuel cell environment over an extended period of timewas evaluated.

(2) Evaluation Results

FIG. 6 is a graph depicting results of evaluating the corrosionresistance and the contact resistance in the simulated fuel cellenvironment by the method as described above.

Referring to FIG. 6, Example 1 and Comparative Example 2 had a corrosioncurrent density of 1 μA/cm² not only at an initial stage but also after1,000 hours, whereas Comparative Example 3 had a corrosion currentdensity exceeding 1 μA/cm² not only at an initial stage but also after along time. It was considered that such a high corrosion current densityof Comparative Example 3 was caused by exfoliation of the CrN layer.

FIG. 7 is a graph depicting results of evaluating the contact resistanceof Examples 1, 4, 8 to 18 and Comparative Examples 1 and 2 exposed to asimulated fuel cell environment for 2,000 hours.

Referring to FIG. 7, Examples 1, 4, 8 to 18 had contact resistancessatisfying the standards of the DOE even after 2,000 hours, whereasComparative Examples 1 and 2 had corrosion resistances exceeding thestandards of the DOE after 2,000 hours.

With regard to such high contact resistances of the comparativeexamples, it was considered that Comparative Example 1 underwentexfoliation of the CrN layer or growth of the passive film above thethickness of the CrN layer, and that Comparative Example 2 underwentcontinuous growth of the passive film. On the other hand, for theinventive examples, it was considered that a metal compound layer formedon the surface of each separator efficiently served as a corrosionresistant and electrically conductive coating layer suppressing thegrowth of the passive film under the metal compound layer.

5. Evaluation and Result of Long-Term Durability of Fuel Cell

(1) Evaluation of Long-Term Durability

Separators, each having a serpentine passage for supplying reaction gas,were used. Each fuel cell was prepared by interposing a membraneelectrode assembly (Model 5710 from Gore Fuel Cell Technologies, Inc.)and a gas diffusion layer (Model 10BA from SGL Co., Ltd.) between theseparators and compressing the same with a predetermined pressure.

Performance of each of the fuel cells was evaluated using a unit cell.NSE Test Station 700 W class was used as a fuel cell operator, andKIKUSUI E-Load was used as an electronic load for evaluating theperformance of the fuel cell. Current cycles of 0.01 A/cm² current for15 seconds and 1 A/cm² current for 15 seconds were constantly applied.

As the reaction gas, hydrogen and air were supplied at a flux with astoichiometric ratio of H₂ to air of 1.5:2.0 according to the electriccurrent after being humidified to a relative humidity of 100%. Theperformance of the fuel cell was evaluated at atmospheric pressure whilemaintaining the temperature of a humidifier and the cell at 65° C. Anactive area was 25 cm² and an operating pressure was 1 atm.

(2) Evaluation Results of Long-Term Durability

FIG. 8 is a graph depicting results of evaluating the long-termdurability of Examples 1, 4 and 8 to 18, and Comparative Examples 1 and2 by the method as described above.

Referring to FIG. 8, although all of the samples generated a voltage ofabout 0.62 V or more at an initial stage, Comparative Example 1generated a decreased voltage of about 0.58 V after 2,000 hours andComparative Example 2 generated a decreased voltage of about 0.57 Vafter 2,000 hours.

Fuel cells including the stainless steel separators of Examples 1, 4 and8 to 18 experienced a minute reduction of voltage less than 0.02 V evenafter 2,000 hours due to superior durability of the stainless steelseparators.

FIG. 9 is a flowchart of a method of manufacturing a stainless steelseparator according to another embodiment of the present invention.

To manufacture a stainless steel separator according to this embodimentof the invention, a stainless steel sheet is prepared in S910.

In this embodiment, the stainless steel sheet is a stainless steel sheetwhich is readily available in the marketplace and contains 16˜28 wt %chromium. Alternatively, the stainless steel sheet may contain about 18wt % chromium.

Specifically, a matrix of the stainless steel separator is a stainlesssteel sheet that comprises 0.08 wt % or less carbon (C), 16˜28 wt %chromium (Cr), 0.1˜20 wt % nickel (Ni), 0.1˜6 wt % molybdenum (Mo),0.1˜5 wt % tungsten (W), 0.1˜2 wt % tin (Sn), 0.1˜2 wt % copper (Cu),and the balance of iron (Fe) and unavoidable impurities. Morespecifically, the stainless steel sheet is an austenite stainless steelsuch as SUS 316L 0.2t.

This operation may include a cleaning process for removing impuritiesfrom the surface of the stainless steel sheet using acid and alkalidegreasers before performing subsequent surface modification andformation of a coating layer.

Next, the surface of the stainless steel sheet is subjected to surfacemodification in S920.

Although the stainless steel sheet contains chromium and nickelcomponents exhibiting high corrosion resistance, the stainless steelsheet is mainly composed of iron (Fe).

As a result, in a natural state, the stainless steel sheet tends toreact with oxygen in air to form an oxide film on the surface thereof.Here, since the oxide film is an insulator, it can cause deteriorationof the overall electrical conductivity of the stainless steel sheet.

Therefore, there is a need for surface modification on the surface ofthe stainless steel sheet which undergoes deterioration in corrosionresistance.

In other words, the surface modification is performed for selectivelyetching only the iron component (Fe) in a superficial layer of thestainless steel sheet.

After the surface modification, the surface of the stainless steel sheetbecomes a Cr-rich passive film. The Cr-rich passive film contains 20˜75wt % chromium and 30 wt % or less iron, and has a (Cr+Ni)/Fe ratio of 1or more as expressed by a ratio of main components in the Cr-richpassive film.

Here, the selective metal dissolution can be accomplished because ironoxide in the superficial oxide film can be easily dissolved in an acidwhereas chromium oxide therein is more stable than the iron oxide anddoes not easily dissolve in acids.

Next, a solution and conditions for the surface modification will bedescribed.

A surface modification solution comprises 5˜20 wt % pure nitric acid(HNO₃), 2˜15 wt % pure sulfuric acid (H₂SO₄), and the balance of water.The surface modification may be performed at 50˜80° C. for an immersionduration of 30 seconds˜30 minutes or less. Here, the surfacemodification may be performed for 30 seconds˜10 minutes or less whileadjusting the concentrations of the nitric acid and the sulfuric acid inconsideration of productivity according to treatment duration.

According to one embodiment of this invention, the surface modificationsolution may be prepared by adding one or both of oxalic acid (C₂H₂O₄)and hydrogen peroxide (H₂O₂) to the aforementioned surface modificationsolution (nitric acid+sulfuric acid) to accelerate a metal dissolutionrate on the surface of the stainless steel sheet.

Further, for the surface modification, an electrochemical process may becarried out by applying an SHE potential of greater than 0.0 to 1.0 V tothe stainless steel sheet which has been immersed in the surfacemodification solution comprising sulfuric acid (H₂SO₄), thereby enablingselective dissolution of Fe in a further reduced period of time.

With the surface modification, a large amount of Fe and a part of Nicontent are selectively dissolved to reduce the amount of Fe in thesuperficial layer of the stainless steel sheet without substantiallydissolving chromium (Cr) therein, so that the chromium and nickelcomponents are concentrated on the superficial layer of the stainlesssteel sheet.

After the surface modification, the Cr-rich passive film may have athickness of 5˜100 nm.

Next, the stainless steel sheet subjected to the surface modificationand having the passive film on the surface thereof is heat-treated inS930.

The heat treatment is performed for the following reasons.

When the stainless steel sheet is subjected to the surface modification,the Cr-rich passive film is formed on the surface of the stainless steelsheet as described above, thereby ensuring superior corrosion resistanceand electrical conductivity at an initial stage.

However, when the surface-modified stainless steel separator is exposedfor long durations to high temperature-high humidity conditions of afuel cell, the passive film is gradually thickened. Since the passivefilm mainly consists of metallic oxides, the stainless steel separatorcan suffer deterioration in electrical conductivity after apredetermined operational period even though the corrosion resistancethereof can be maintained.

Accordingly, the separator for the fuel cell can be prepared to havesuperior corrosion resistance and electrical conductivity not only at aninitial operating stage of the fuel cell but also after long-termoperation thereof through heat treatment for suppressing the growth ofthe passive film even after long-term operation while ensuring bothsuperior corrosion resistance and electrical conductivity on the Cr-richpassive film.

The heat treatment may be performed under vacuum, in air, or in an inertgas (for example, nitrogen, argon, helium, hydrogen, etc.) atmosphere ata temperature of 100˜300° C., and preferably at a temperature of100˜200° C.

Heat treatment at a temperature of 100° C. or less provides aninsignificant effect upon the stainless steel sheet. On the other hand,if the heat treatment is performed above 300° C., oxidation occurs onthe surface of the stainless steel sheet, deteriorating the propertiesthereof, and it is also undesirable in view of manufacturing costs.

Although a heat treatment period is not specifically limited, the heattreatment is advantageously performed for 3 minutes or more. Further,the heat treatment may be performed for 1 hour or less when taking intoconsideration a temperature increasing time and costs.

In all examples of the present inventions described below, the heattreatment was performed for 30 minutes.

The heat treatment may be performed in a batch-type manner or acontinuous line manner in a furnace.

The stainless steel separator for fuel cells manufactured by the methodaccording to this embodiment of the invention, that is, through thesurface modification and heat treatment, has a corrosion current densityof 1 μA/cm² or less and a contact resistance of 20 mΩ·cm² or less onboth surfaces, which satisfy the standards of the DOE.

EXAMPLES AND COMPARATIVE EXAMPLES

TABLE 4 Surface modification Time (current Heat treatment CorrosionTemp. density) Time Temp. rate CR Process (° C.) (min) Comp. Atm. (min)(° C.) (μA/cm²) (mΩ · cm²) E19 IM 60  3 15% HNO₃ + Vacuum 30 200 0.5412.2 10% (1 * 10⁻³ torr) H₂SO₄ E20 EC 60 (100) 1M H₂SO₄ Vacuum 30 1000.62 13.8 (1 * 10⁻³ torr) E21 EC 60 (100) 1M H₂SO₄ N₂ 30 200 0.61 12.8E22 EC 60 (100) 1M H₂SO₄ N₂ 30 100 0.67 14.5 E23 EC 60 (100) 1M H₂SO₄ Ar30 200 0.59 12.5 E24 EC 60 (100) 1M H₂SO₄ Ar 30 100 0.64 14.1 E25 EC 60(100) 1M H₂SO₄ Air 30 100 0.57 17.5 E26 EC 60 (100) 1M H₂SO₄ Air 30 2000.49 17.6 E27 EC 60 (100) 1M H₂SO₄ Air 30 300 0.43 17.8 CE4 EC 60 (100)1M H₂SO₄ — — — 0.95 17.5 CE5 EC 60 (100) 1M H₂SO₄ Air 30 400 0.35 23.3CE6 EC 60 (100) 1M H₂SO₄ Air 1 100 0.95 17.4 CE7 EC 60 (100) 1M H₂SO₄Air 1 200 0.94 17.4 CE8 EC 60 (100) 1M H₂SO₄ Air 2 300 0.94 17.3 E =Example, CE = Comparative Example, IM = immersion process, EC =electrochemical process, Atm.: atmosphere, CR: Contact resistance

Table 4 shows corrosion currents and contact resistances of stainlesssteel separators of Examples 19 to 27 and Comparative Examples 4 to 8prepared using stainless steel 316 L as matrices of the stainless steelsheet separators under different conditions for surface modification(temperature, time, and composition of solution), heat treatment(atmosphere and temperature) by an immersion process and anelectrochemical process.

Specifically, Examples 19 and 20 were subjected to surface modificationand heat treatment in a vacuum of 1×10⁻³ torr. Examples 21 and 22 weresubjected to surface modification and heat treatment in a nitrogenatmosphere. Examples 23 and 24 were subjected to surface modificationand heat treatment in an argon atmosphere as a Group 0 inert gasatmosphere. Examples 25 to 27 were subjected to surface modification andheat treatment in air.

Comparative Example 4 was subjected to surface modification without heattreatment, and Comparative Example 5 was subjected to surfacemodification and heat treatment in air at a temperature of 400° C. whichdid not satisfy the conditions of the present invention. ComparativeExamples 6 to 8 were subjected to surface modification and heattreatment in air for 1 minute and 2 minutes, respectively, which did notsatisfy the conditions of the present invention.

1. Measurement of Contact Resistance

The contact resistances of samples 500, that is, stainless steel sheets,of the inventive and comparative examples shown in Table 4 were measuredusing the contact resistance tester 50 as shown in FIG. 5.

2. Measurement of Corrosion Current Density

The corrosion current density was measured by the same method as inExamples 1 to 18.

3. Analysis of Measurement Results of Corrosion Current Density andContact Resistance

Referring to Table 4, it can be understood that, when the samples weresubjected to the surface modification and heat treatment satisfying theconditions of the present invention as in the inventive examples, all ofthe samples had a corrosion current density of 0.5˜0.7 μA/cm² and acontact resistance of 12˜18 mΩ·cm², all of which satisfy the standardsset by the DOE.

Comparative Example 4 subjected to the surface modification without theheat treatment had a contact resistance of 17.5 mΩ·cm² and a corrosioncurrent density of 0.95 μA/cm², and Comparative Example 5 subjected tothe surface modification and the heat treatment in air at a highertemperature than that of the present invention had a contact resistanceof 23.3 mΩ·cm² and a corrosion current density of 0.35 μA/cm².Comparative Examples 6 to 8 subjected to the surface modification andthe heat treatment in air for 1 minute had a contact resistance of17.3˜17.4 mΩ·cm² and a corrosion current density of 0.94˜0.95 μA/cm².

Here, although Comparative Examples 4, 6 to 8 had the contact resistanceand the corrosion current satisfying the standards of the DOE, thesevalues were merely initial values before long-term operation of a fuelcell, and it can be understood that a difference between the inventiveexamples and Comparative Example 4 is significant in evaluation oflong-term durability of the fuel cell described below.

4. Evaluation and Results of Corrosion Resistance and Contact Resistancein Simulated Fuel Cell Environment

(1) Evaluation of Corrosion Resistance and Contact Resistance in aSimulated Fuel Cell Environment

The variation in corrosion resistance and contact resistance in asimulated fuel cell environment was evaluated by the same method as inExamples 1 to 18.

(2) Evaluation Results of Contact Resistance and Long-Term CorrosionResistance in a Simulated Fuel Cell Environment

FIG. 10 is a graph depicting evaluation results of the contactresistance measured by the aforementioned method in the simulated fuelcell environment.

Referring to FIG. 10, Examples 19, 21, 23 and 26 had a contactresistance of 20 mΩ·cm² or less not only at an initial stage (0 hour)but also after 2,000 hours, indicating that the contact resistancethereof was substantially maintained. Conversely, Comparative Examples4, 6 to 8 had a contact resistance of 17.3˜17.5 mΩ·cm² at an initialstage as described above, but had a contact resistance above 40 mΩ·cm²after 2,000 hours.

FIG. 11 is a graph depicting results of evaluating corrosion currentdensity of Examples 19 and 21 and Comparative Example 4 exposed to thesimulated fuel cell environment for 2,000 hours as described above.

Referring to FIG. 11, all of the samples of Examples 19 and 21 andComparative Example 4 had corrosion current densities less than or equalto the standards of the DOE not only at an initial stage but also after2,000 hours.

Therefore, it can be seen that, when the stainless steel sheet wassubjected to the surface modification without heat treatment, thestainless steel separator could maintain corrosion resistance but wassignificantly increased in surface resistance after the long-termoperation in the simulated fuel cell environment.

5. Evaluation and Results of Long-Term Durability of Fuel Cell

(1) Evaluation Method of Long-Term Durability

The long-term durability was evaluated by the same method as in Examples1 to 18.

(2) Evaluation Results of Long-Term Durability

FIG. 12 is a graph depicting results of evaluating the long-termdurability of Examples 19, 21, 23 and 26 and Comparative Examples 4, 6to 8 by the method as described above.

Referring to FIG. 12, fuel cells of Comparative Examples 4, and 6 to 8generated a voltage of 0.6 V or more at an initial stage, but generateda decreased voltage of about 0.57 V after 2,000 hours.

On the other hand, all of the fuel cells including the stainless steelseparators of Examples 19, 21, 23 and 26 generated a voltage of 0.62 Vor more at an initial stage, and experienced a minute reduction ofvoltage less than 0.02 V even after 2,000 hours due to superiordurability of the stainless steel separators.

As apparent from the above description, the stainless steel separatorfor fuel cells manufactured by the method according to an embodiment ofthe invention has superior corrosion resistance and electricalconductivity not only at an initial stage but also after long-term usein operational conditions of the fuel cell.

Further, the method according to the embodiment of the present inventionenables surface modification for achieving superior properties even witha general inexpensive stainless steel sheet, thereby loweringmanufacturing costs of the stainless steel separator.

The stainless steel separator for fuel cells manufactured by the methodaccording to the embodiment of the invention has a corrosion currentdensity of 1 μA/cm² or less and a contact resistance of 20 mΩ·cm² orless on both surfaces of the separator.

Although some embodiments have been provided in conjunction with theaccompanying drawings to illustrate the present invention, it will beapparent to those skilled in the art that the embodiments are given byway of illustration, and that various modifications and changes can bemade without departing from the spirit and scope of the presentinvention. Accordingly, the scope of the present invention should belimited only by the accompanying claims.

What is claimed is:
 1. A stainless steel separator for fuel cells,comprising: a stainless steel sheet having a matrix structure; a Cr-richpassive film directly formed on a surface of the stainless steel sheet,and containing 20˜75 wt % chromium; and a coating layer disposeddirectly on the Cr-rich passive film, and having a thickness of 30˜300nm, wherein the coating layer is a metal boride layer (MB_(z)), andwherein 0.5≦z≦2.
 2. The stainless steel separator according to claim 1,wherein the coating layer is a continuous film disposed on the passivefilm.
 3. The stainless steel separator according to claim 1, wherein themetal (M) of the coating layer is at least one selected from chromium(Cr), titanium (Ti), zirconium (Zr), and tungsten (W).
 4. The stainlesssteel separator according to claim 1, wherein the passive film has a(Cr+Ni)/Fe ratio of 1 or more in terms of atomic weight ratio.
 5. Thestainless steel separator according to claim 1, wherein the stainlesssteel sheet comprises 0.08 wt % or less carbon (C), 16˜28 wt % chromium(Cr), 0.1˜20 wt % nickel (Ni), 0.1˜6 wt % molybdenum (Mo), 0.1˜5 wt %tungsten (W), 0.1˜2 wt % tin (Sn), 0.1˜2 wt % copper (Cu), and thebalance of iron (Fe) and unavoidable impurities.
 6. The stainless steelseparator according to claim 1, wherein the stainless steel separatorhas a corrosion current density less than 1 μA/cm² and a contactresistance less than 20 mΩ·cm² on both surfaces thereof.